1. Field of the Invention
This invention relates to an error diffusion scheme combined with traditional halftoning which can be used for printing black and white as well as color images.
2. Discussion of Related Art
Several techniques have been proposed for combining traditional halftoning, and error diffusion over the last few years, such as is disclosed in U.S. Pat. No. 5,226,096 to Fan, U.S. Pat. No. 5,243,443 to Eschbach and U.S. Pat. No. 5,321,525 to Hains. The rationale behind such combinations is to reduce coarse quantization artifacts without sacrificing resolution. Coarse quantization artifacts are visible in image areas where a scene has little variation. This is also known as "banding." Banding is due to a limited number of output gray levels being available. The banding artifacts generally increase with decreasing cell size, which is identical to a decrease in the number of levels that can be represented by a halftone cell. The techniques mentioned above by Fan and Hains partition an image into disjoint areas which are usually rectangular blocks. Thereafter, the image is then processed block-by-block. This processing typically includes, as a first step, ordered dithering or halftoning after which block-based error diffusion techniques are used to reduce quantization errors. A major constraint of block-based error diffusion techniques is that the blocks are required to cover a halftone cell. It is well known that digital halftone threshold matrices can be represented by periodically repeated rectangular blocks using Holladay's method; Thomas M. Holladay, An Optimum Algorithm for Halftone Generation for Displays and Hard Copies', Proc. Society for Information Display, Vol. 21, No. 2, 1980, pp. 185-192. Consequently for applications which involve printing black and white images the constraint mentioned above does not cause much of a problem since such printing involves the use of 45.degree. screens, which have a Holladay block, width-to-height ratio of 2. However, the same is not true for printing color images which typically involve the use of arbitrary screen orientations, e.g., 15.degree. or 75.degree. screens. Typically, such screens have blocks which have very large or very small width-to-height ratios which makes it difficult to use block-based error diffusion.